To connect more than one semiconductor device, a number of semiconductor chips are stacked vertically one on the other and connected together through electrical terminal contacts on the upper sides and lower sides. For this, electrically conductive connections must be made from the relevant upper side of the chip to the lower side through the substrate. This usually is done by etching holes in the substrate and then filling them with an electrically conductive material, usually a metal. If the electrical conductors made in this way do not reach the backside of the substrate, the substrate is thinned from the backside by grinding until the conductive material of the contact hole filling is exposed and in this way the via is produced. Metal layers can be applied to the surfaces of the substrate and structured into electric terminals. When the chips are stacked, the matching terminal contact areas are arranged on top of each other and, electrically connected together permanently, for example by soldering (J. Vardaman, “3-D Through-Silicon Vias Become a Reality”, Semiconductor International, Jun. 1, 2007).
Vias with diameters of typically about 50 μm to 100 μm can be made by etching larger recesses with slanted side walls, for example, using KOH. A metal layer deposited in the recess is exposed from the opposite surface of the wafer and provided with a contact there. Current conventional methods are described in U.S. Pat. No. 6,323,546, U.S. Pat. No. 6,352,923 and U.S. Pat. No. 6,110,825.
The metal layer of the via is covered with a passivation layer. It was established that cracks can occur in the passivation layer, or the passivation layer can separate from the metal layer.
U.S. Pat. No. 7,402,515 describes a via in which the contact hole is filled with a metal and a buffer layer is present to compensate for mechanical stresses between the semiconductor material and the metal. Materials mentioned for the buffer layer are silicones, acrylates, especially PMMA (polymethyl methacrylate), polyimide, benzocyclobutene (BCB), epoxy resin, polyparaxylene, fluorocarbons, especially PTFE (polytetrafluoroethylene), organic silicon compounds like PDMS (polydimethylsiloxane), polyesters and polyolefins.